fix incorrect usage of dbt_cube model property

Author: AvilaJulio

docs/pages/guides/dbt.mdx

@@ -259,7 +259,7 @@ cubes:
   - {{ model.as_cube() }}
 
     dimensions:
-      {% for column in model.columns() %}
+      {% for column in model.columns %}
       - name: "{{ column.name }}"
         sql: "{{ column.sql }}"
         type: "{{ column.type }}"